Heretofore, the most successful antifouling coatings have comprised a relatively inert organic binder with a biocidal pigment which is leached from the paint. Zinc oxide has been used in the past in antifouling coating compositions as a white pigment, to impart flexibility and hardness to the coating and/or as an insolubilizing agent. Zinc oxide is not recognized as useful as a primary toxicant, probably because it is only sparingly soluble in neutral water. Thus, while heretofore zinc oxide has been widely used in antifouling coatings, its role is as a supporting compound to recognized biocidal agents.
Previously it was thought that coating compositions containing zinc oxide levels higher than 20 wt % greatly reduced the antifouling properties and life of the coating unless small levels of a very toxic material were also included. This is because higher levels imply replacing the primary toxicant with zinc oxide and the effectiveness of the coating was believed to be directly related to the level of the primary toxicant in the coating. Levels higher than 20 wt % have in the past been limited to coating compositions containing extremely toxic triorganotin moieties where only relatively small amounts of primary toxicant are necessary. (See, for example, U.S. Pat. No. 4,139,515 (Dennington) issued Feb. 13, 1959.)
Heretofore, ultraviolet light has been used to sterilize surfaces. However, the solution to the problem of using visible light to render surfaces toxic to pestiferous organisms has not been known in the art. A solution to the problem would be highly desired since visible light is more commonly available than is ultraviolet light and is a nontoxic, renewable resource.
Use of visible light to produce phototoxic surfaces would be especially useful for coatings on fish nets and boat bottoms. Equipment submerged in natural waters at depths of more than one meter only receives significant amounts of light energy in the blue-green region of the electromagnetic spectrum at around 500 nm. Thus a phototoxic surface which utilizes blue and green light would be highly desired by boat owners and aquaculturists to name a few.
This invention relates to utilizing visible light to photochemically synthesize an effective concentration of hydrogen peroxide by in situ reduction of oxygen on zinc oxide. It is known that photolysis of aqueous, aerated solutions containing zinc oxide pigment leads to the formation of hydrogen peroxide only when exposed to ultraviolet light of wavelengths greater than 400 nm and thus has been thought to be ineffective for producing phototoxic surfaces.
Hydrogen peroxide is a known toxicant but prior methods for generating hydrogen peroxide on surfaces are inefficient and expensive. Such techniques have included applying cathodic voltage and current to a conducting or semiconducting surface to produce at or near the surface an effective concentration of hydrogen peroxide.
The very success of toxic antifouling coatings based on biocidal pigment leaching has led to their over use. This has so polluted the environment that lead, mercury, and triorganotin based coatings are now banned in most parts of the world. Copper-based paints are still permitted, but they are classified as pesticides needing strict government controls for testing and use.
The leaching of soluble copper salts into navigable waters from pleasure craft, nuclear power plants, and the like is of great concern to State and Federal Authorities especially in areas of high concentrations of polluting sources. Present antifouling paints are a major cause of such concern. Copper-based paints now must exhibit limited release rates of copper to a point where for many applications they are ineffective.
Heretofore, toxic antifouling coatings relied entirely on the leaching into the environment of poisons contained within the coating. Over time, even the best heretofore available antifouling coating loses its toxicant and becomes ineffective.